Method for correcting a control of an optical scanner and the optical scanner

ABSTRACT

In a method for correcting a control of an optical scanner ( 14 ) which has a beam deflecting element ( 31 ) for deflecting a beam of optical radiation and a drive unit ( 30, 30 ′) for moving the beam deflecting element ( 31 ), said drive unit deflecting a beam of optical radiation directed at the beam deflecting element ( 31 ) according to a predetermined setpoint movement using at least one parameter and/or a transfer function, preferably optical, said parameter or transfer function being used to control or regulate the system. In a determination step at least one instantaneous value of a drive unit transfer function that reproduces the response of the drive unit ( 30, 30 ′) to a predetermined setpoint movement or a change in a setpoint movement is ascertained for at least one frequency, and in a correction step at least one parameter and/or the transfer function is corrected as a function of the instantaneous value of the drive unit transfer function.

The present invention relates to a method for correcting a control of anoptical scanner and an optical scanner.

Optical scanners, which are understood below to include in particulardeflecting devices for controlled deflection of a beam of opticalradiation that can be controlled in particular by preselecting asetpoint movement or can be controlled by trigger signals, are used invarious areas of technology. Laser scanning microscopes constitute animportant area for use of such optical scanners. With laser scanningmicroscopes, a sample is scanned with a laser beam focused as a rule ina spot on a layer of the sample, whereby the focus is confocally mappedon a detection device. To deflect the laser beam to a predeterminedposition on the sample and to deflect the detection radiation emanatingfrom the focus to the detection device, a deflection device, i.e., anoptical scanner is used, permitting controlled deflection of the laserbeam, i.e., the detection radiation. For detection of an image of alayer of the sample, the laser beam and/or its focus is guided line byline over the sample from a first end position to a second end positionand then back, with the beam being deflected in each of the endpositions in a direction orthogonal to the direction of movement in theline, so that the focus jumps to the next line. The focus is moved inthe most linear and uniform possible movement in the line, so that bydetection of the detection radiation in constant intervals of time, apixel representation of an image in which the pixels, which are alsoarranged in a matrix, are assigned to the locations, is obtained, inwhich the pixels are then also arranged in the form of a matrix so theyare equidistant. Therefore, an undistorted image is obtained only whenthe movement of the focus corresponds precisely to a uniform linearmovement. The requirements of accurate control of an optical scannerused in a laser scanning microscope are very high.

To obtain a high accuracy, as described in DE 197 02 752 C2, forexample, by means of a position sensor, the position of the drive and/orthe mirror of the scanner may be detected in the form of a check-backsignal and/or a position signal on the basis of which deviations from asetpoint position are corrected. However, this procedure is not accurateenough for high scanning speeds. Electronic processing of check-backsignals and detection signals leads to distortion of the signals and tophase differences between the signals. In addition, owing to deformationof the scanner wave by means of which the drive movement is transmittedto the mirror, and deformations in the scanner mirror itself, theacknowledged position does not correspond to the actual position of themirror. The actual position of the deflected beam thus does notcorrespond to the desired setpoint position.

It is therefore conceivable to perform an optical calibration of thescanner in which the control of the scanner is adjusted so that theactual movement of the focus corresponds to the setpoint movement asaccurately as possible.

However, even when there is very good optical calibration, errors in theimage, e.g., a “fraying” of vertical lines or distortion of the imagegeometry may occur after an extended period of time with a bidirectionalscan due to wear on the scanner, in particular a drive unit of thescanner, because the scanner does not respond to triggering in the sameway as in optical calibration. However, optical calibration is toocomplicated and expensive to be able to perform it frequently.Therefore, the object of the present invention is to provide a methodfor correcting a control of an optical scanner that is simple and quickto perform and to provide an optical scanner whose control can becorrected easily and quickly.

This object is achieved by a method for correcting a control of anoptical scanner having a beam deflecting element for deflecting a beamof optical radiation and a drive unit for moving the beam deflectingelement, which moves the beam deflecting element out of a predeterminedsetpoint movement according to trigger signals which are generated usingat least one parameter that is used for control or regulation and/orusing a transfer function, preferably optical, wherein, in adetermination step, at least one instantaneous value of a drive unittransfer function is determined, said drive unit transfer functionreproducing the response of the drive unit to trigger signals that aredetermined for at least one frequency from a preselected setpointmovement or a change in a setpoint movement, and in a correction step atleast one parameter and/or the transfer function is corrected as afunction of the instantaneous value of the drive unit transfer function.This object is also achieved by an optical scanner with a beamdeflecting element for deflecting a beam of optical radiation, a driveunit for movement of the beam deflecting element, which moves the beamdeflecting element according to trigger signals, and a scan control unitconnected to the drive unit for transmitting trigger signals forcontrolling the drive unit by generating trigger signals, said unitbeing designed so that corresponding trigger signals can be generatedusing at least one parameter that is used for control or regulationand/or one transfer function, preferably optical, of a predeterminedsetpoint movement, and for correcting the control of the scanner, atleast one instantaneous value of a drive unit transfer function can bedetermined for at least one frequency in a determination step, saiddrive unit transfer function reproducing the response of the drive unitto trigger signals generated from a predetermined setpoint movement or achange in a setpoint movement, and in a correction step at least oneparameter and/or the transfer function can be corrected as a function ofthe instantaneous value of the drive unit transfer function.

The optical scanner has a drive unit, a beam deflecting element that ismovable by the drive unit, e.g., a mirror or a prism, and a scan controlunit for generating trigger signals for the drive according to asetpoint movement which is preferably designed for execution of theinventive method. The drive unit itself has a drive, e.g., an electricmotor, a piezo drive or a galvanic drive. The scan control unit may bean electric and/or electronic circuit for delivering control signalsaccording to a predetermined setpoint movement to the drive. The scancontrol unit or controller and/or the electronic circuit may have inparticular a processor, e.g., a digital signal processor which generatestrigger signals for the drive or another control or regulating device ofa higher priority than the drive, generating the signals from apreselected setpoint movement which may be defined by parameters, e.g.,as a distance-time function for the focus of the beam that is deflectedand/or is to be deflected by the scanner in a plane. Depending on thetype and intended purpose of the scanner, the setpoint movement mayessentially be based on the movement of the beam deflected by thescanner, either in the form of angles or as a point in a reference planeor based on the movement of the beam deflecting element.

To generate the trigger signals, the scan control unit, i.e., thecircuit and/or the processor, uses at least one parameter that is usedfor control or regulation. Such parameters may be in particularparameters that reproduce the setpoint movement. In addition, they mayuse a transfer function and/or a frequency response that reflects thecorrelation between the setpoint movement and the actual movement.

In the simplest case, the transfer function used may be the drive unittransfer function which reproduces the relationship between the setpointmovement and/or the trigger signals corresponding to the setpoint unitand the position of the drive or a mechanical coupling element, e.g., ashaft that is driven by the drive and moves the beam deflecting element.The drive unit transfer function preferably also includes thosecomponents that are used by the measurement of the position of the driveor the coupling element, in particular in the case when this positionregulator is used. For example, it may include the effect of a positionsensor and/or a feedback sensor for detecting the position of the driveand/or the coupling element and delivering suitable trigger signals andthe following check-back signal processing including an analog-digitalconversion, if the latter is provided at all.

Preferably, however, the optical transfer function is used, reproducingthe relationship between the setpoint movement of the beam to bedeflected and/or the corresponding trigger signals and the movementinduced by the scanner and thus also the properties of the couplingelement and beam deflecting element.

The transfer function may in particular be used by the scan control unitto generate predistorted trigger signals corresponding to the setpointmovement, these trigger signals being predistorted in such a way thatthe transfer and/or implementation by the drive leads to the desiredsetpoint movement with an accurate determination of the transferfunction.

The invention is based on, among other things, the finding that, firstof all, for accurate control of a scanner, the most accurate possibleknowledge of the transfer function of the scanner is required and that,secondly, the transfer function can vary over a period of time. Inaddition, it has been recognized that the optical transfer function canbe divided into two components that are linked together, namely thedrive unit transfer function and another component. The other componentmay describe, among other things, the transfer of the movement of thedrive via the movement of the beam deflecting element, which may also bedeformed by the movement, to the movement of the beam to be deflected.The drive unit transfer function is preferably defined for performingthe method so that it can be measured easily. If a position sensor isused to detect the position of the drive, then the drive unit transferfunction advantageously also describes the properties of all units thatconvert raw signals of a sensor element of the position sensor intovalues that can be used for correction. For example, electronic circuitsfor processing, in particular for conditioning the raw signals andoptionally also an analog-digital converter may belong here. Thesecomponents may be arranged so they are offset physically from the actualsensor.

The measurement of the additional component is usually complex and mayrequire, for example, the use of optical auxiliary means such asreference samples in particular. However, it has been recognized thatthis component changes very little over a period of time in comparisonwith the drive unit transfer function.

The drive unit transfer function can be determined electrically, i.e.,electronically, relatively easily, quickly and accurately, so it issufficient for an improvement in the calibration and/or control of thescanner to determine only this function and/or at least a componentthereof and to correct the control using at least one component therebyascertained. By linking the function thereby ascertained and/or at leastone component thereof to the known prevailing transfer function, thelatter may be corrected easily and quickly. To do so, it may besufficient to perform the correction for only a predetermined frequencywhich may be selected as a function of the setpoint movement and theproperties of the transfer function. In particular in the case of aperiodic setpoint movement, the predetermined frequency may be thefundamental frequency of the setpoint movement or one of the harmonicfrequencies, i.e., one of the multiples of the fundamental frequency.

Therefore, the invention offers the great advantage that control of thescanner can be corrected easily and quickly so that when there arerepeated corrections even over long periods of time, triggeringaccording to a predetermined setpoint movement will lead to an actualmovement that corresponds to the setpoint movement with a very highprecision. Since the correction can be performed very quickly, it may beautomated in particular and may be accomplished in a manner that ishardly perceptible by the user.

In the determination step, the drive unit transfer function mayessentially be ascertained any number of times. In this method, however,the instantaneous value of the drive unit transfer function ispreferably determined for the preselected frequency by ascertaining anactual position of a drive or a mechanical coupling element of the driveunit as a function of the setpoint movement and/or the change in thesetpoint movement or the trigger signals ascertained therefrom. In thecase of the optical scanner, the drive unit therefore preferably has astepping motor for moving the beam deflecting element and/or a positionsensor for sensing the position of the drive or a mechanical couplingelement of the drive unit and furthermore the scan control unit isdesigned so that in the ascertaining step the position of the steppingmotor or a position signal of the position sensor is used to ascertainthe instantaneous value of the drive unit transfer function. Thisembodiment has the advantage, especially when using a position sensor,that the position signal may be used in ordinary operation of regulatingthe position of the motor, to which end the drive unit then preferablyhas a position regulator that triggers the motor. The use of a steppingmotor allows a simple means of acquisition of the position of the driveand/or the coupling element. A shaft in particular may be used as thecoupling element. In any case, this yields a particularly simple designof the scanner.

It may fundamentally be sufficient for the method to be used only asneeded. The correction method may then be started manually by the user.However, the method is preferably performed automatically, to which endthe optical scanner and/or its scan control unit is designedaccordingly.

In a first preferred variant of the method, the determination step andthe correction step are repeated in preselected intervals of time. Theoptical scanner preferably has a time switching device which repeatedlytriggers the scan control unit in predetermined intervals of time sothat it executes the determination step and the correction step. Theintervals of time may be selected in particular as a function of theexpected change in the properties of the scanner and the requiredpositional accuracy and/or precision in movement. For example, themethod may be performed every week or every month.

However, it is also possible to repeat the determination step and thecorrection step when a preselected total operating time of the scannersince the last correction has been reached. This has the advantage thatthe correction is performed as a function of the wear which isdetermined by use, so that even when usage is irregular, a correction isperformed early enough.

According to another preferred variant, the determination step and thecorrection step may be performed in this method each time the scanner isactivated or at a predetermined interval of time after each activationof the scanner. In the case of the optical scanner, the scan controlunit is therefore preferably further designed so that the determinationstep and the correction step are performed each time the scanner isactivated or at a predetermined interval of time after each activation.The interval of time may be selected in particular so that the scanneror a device containing the scanner will have warmed up after this periodof time has elapsed and will have stable operating conditions. Thisembodiment has the advantage that with a suitable choice of thepredetermined interval of time, the correction is performed after awarm-up phase of the scanner and/or a device containing the scanner andthus when conditions are stable. This increases the reliability of thecorrection.

Essentially it may be sufficient to perform the correction only for apredetermined frequency. In this method, however, it is preferable forthe determination step and the correction step to be repeated and forthe frequency which is used in these steps to change with eachrepetition. To do so, preferably in the optical scanner, the scancontrol unit is further designed so that it repeatedly executes thedetermination step and the correction step and changes the frequencyused in these steps with each repetition. In particular, the fundamentalfrequency and the harmonic frequencies that are used, i.e., multiples ofthe fundamental frequency, may be used as the frequency one after theother in a typical periodic setpoint movement with a fixed fundamentalfrequency. This has the advantage that after a sufficient number ofrepetitions, the entire transfer function and/or the parameters used forcontrol and/or regulation is updated.

In addition, the determination step and the correction step maypreferably be performed for several preselected frequencies in thismethod. With the optical scanner, the scan control unit is preferablyfurther designed so that the determination step and the correction stepare performed for multiple preselected frequencies. This means that whenthere is a correction, corrections for multiple frequencies may beperformed essentially simultaneously and/or in direct succession so thata more comprehensive and/or more accurate correction can be achieved inan advantageous manner.

In the correction step, the correction may be performed in various ways.In this method it is preferable for a deviation in the instantaneousvalue of the drive unit transfer function to be determined in thecorrection step, said deviation being determined in relation to acorresponding value of the drive unit transfer function that wasdetermined previously and was determined within a predetermined intervalof time prior to or after the determination of the value of the opticaltransfer function currently being used at the frequency and it ispreferable for the deviation to be used for the correction. With theoptical scanner it is preferable here for the scan control unit to befurther designed so that in the correction step, a deviation in theinstantaneous value of the drive unit transfer function from acorresponding value of the drive unit transfer function that was savedpreviously and was determined within a preselected interval of timeprior to or after the determination of the value of the optical transferfunction currently being used at the frequency and for the deviation tobe used for the correction. The preselected interval of time ispreferably selected so that there is no significant change in the driveunit transfer function nor need any change be expected in this intervalof time. The drive unit transfer function is preferably performedimmediately before or after the determination of the transfer function,in particular the optical transfer function. Depending on how thetransfer function is represented, the deviation may be given as aquotient of the two values or as a difference. If the transfer functionis represented in the form of a Fourier transform, e.g., as a complexfunction, or by two amplitude components for a sine function and cosinefunction, then the deviation may be given by the quotient of thecorresponding values. In the case of a Fourier transform in the form ofamplitudes and phases for sine or cosine functions of a Fourier sum, thedeviation for the amplitude may be given in the form of a quotient andthe deviation in the phases may be given in the form of a difference.This embodiment has the advantage that the correction itself is easy toperform after determining the deviations.

In another preferred embodiment, in this method the correction for atleast one frequency is performed at least in the correction step and amodel for the frequency dependence of at least one parameter to becorrected and/or the transfer function to be corrected is determined forcorrection for other frequencies. With the optical scanner, the scancontrol unit is therefore preferably further designed so that in thecorrection step, the correction is performed for at least one frequencyand a model for the frequency dependence of the parameter to becorrected and/or the transfer function to be corrected is determined forcorrection for other frequencies. This embodiment has the greatadvantage that a correction can be performed over the entire requiredfrequency range despite the determination of a value of the drive unittransfer function for only one or only a few frequencies, which cantherefore take place rapidly. The model may be a polynomial model, forexample, when using values for multiple frequencies. However, it is alsopossible to use the characteristic of the drive unit transfer functiondetermined as a function of frequency as the model in determining thetransfer function or to use a model that is derived theoretically.

The invention is also used to particular advantage in laser scanningmicroscopes, because these microscopes require scanners that operatewith a very high precision to obtain the least possible distortion inimages at a high scanning speed. Therefore, the present invention alsorelates to a laser scanning microscope having an inventive opticalscanner.

The invention will be explained in further detail below as an example onthe basis of the drawings, in which:

FIG. 1 shows a schematic diagram of a laser scanning microscope,

FIG. 2 shows a schematic block diagram of components of the laserscanning microscope in FIG. 1 which are relevant for an optical scannerof the laser scanning microscope,

FIG. 3 shows a simplified distance-time diagram for a setpoint movementand an approximate setpoint movement,

FIG. 4 shows a block diagram for the generation of trigger signals for adrive of the scanner in FIG. 2,

FIG. 5 shows a simplified flow chart for a method for determining anoptical transfer function,

FIG. 6 shows a simplified flow chart for substeps of the method depictedin FIG. 5,

FIG. 7 shows a schematic diagram of a pixel design of an image detectedwith the laser scanning microscope in FIG. 1,

FIG. 8 shows a schematic diagram of a test sample with setpointpositions of line structures and with actual positions of the linestructures detected in a back-and-forth movement,

FIGS. 9 a, b shows diagrams in which deviations between setpointpositions and actual positions of line structures for the back-and-forthmovement have been plotted as a function of the pixel numbers, and

FIG. 10 shows a diagram in which the deviations in FIGS. 9 a and 9 bhave been plotted as a function of time.

FIG. 1 shows a simplified diagram of a laser scanning microscopeaccording to a first preferred embodiment of the invention that servesto examine an object and/or a sample 2 via an illumination section 3 foremission of a collimated illumination beam 4, a deflection section 5 forcontrolled deflection of the illumination beam 4, a mapping optics 6 anda detection unit 7 for detecting recorded image data for at leastpartial images of the object 2 which are mapped by the mapping optics 6on the acquisition device 7. A control and analyzer device 8 isconnected to the acquisition device 7 and to a display device 9 in theform of a color monitor.

With this laser scanning microscope 1, the sample 2 is mapped byscanning it with the illumination beam 4 in a layer of the sample 2 tobe mapped, said beam being focused in the layer and having anapproximately dot-shaped cross section there. The illumination section 3serves to emit the collimated illumination beam 4 with a predeterminedbeam profile and cross section and to this end has a source 10 foroptical radiation, e.g., a laser, along an illumination beam path andhas a fine aperture 12 (pinhole aperture) in a conjugated plane with theposition of the focus of the illumination beam 4 on the sample 2.

The illumination radiation 4 emitted from the illumination section 3passes through a beam splitter 13 into the deflection section 5 forcontrolled deflection of the illumination beam 4 passing through thebeam splitter 13. The deflection section 5 therefore has a deflectingdevice connected to the control and analyzer device 8 and/or has anoptical scanner 14 according to a first preferred embodiment of theinvention and has a scanning lens 15.

The optical scanner 14 is designed to pivot the illumination beam 4 in aplane orthogonally to the direction of the illumination beam 4 upstreamfrom the deflection device, i.e., orthogonally to the plane of thedrawing in FIG. 1. To this end, it has two drive units which can betriggered by the control and analyzer device 8, driving to drives thatcan be triggered independently of one another by the scan control unit35, of which only the drive 30 for movements in x-direction is shown inFIG. 2 and two beam deflecting elements, each pivotable about an axis bymeans of the drives, namely in this example the mirror, of which FIG. 2shows only the mirror 31 that is driven by the drive 30 via a couplingelement 30′, namely a shaft. The drive units are connected to a jointscan control unit 35 for delivering trigger signals according to asetpoint movement in the x-direction and a setpoint movement in they-direction.

Galvanic drives are used in this example. The axes of rotation aboutwhich the mirrors can be rotated run orthogonally to one another,whereby the mirror 31 deflects the illumination beam 4 in x-direction ina controlled manner according to the triggering of the drive 30. Theother mirror causes a deflection in y-direction. In addition acapacitive position sensor is provided for each of the drives fordetecting the position of the respective drive 30 or of the couplingelement 30′ and/or the mirror and emitting corresponding check-backsignals, and a position regulator that is connected to the positionsensor and to the control and analyzer device 8 for transmitting triggersignals, of which FIG. 2 again shows only the position sensor 32 and theposition regulator 33. In response to a trigger signal, the positionregulator 33 regulates the position of the drive 30 to a positiondefined by the trigger signal and does so by using position andcheck-back signals from the position sensor 32 so that the position canbe adjusted with great accuracy, at any rate when there is a slowmovement, but can be adjusted as a function of the properties of theposition sensor 32.

The scan control unit 35 may have at least one digital signal processorfor generating trigger signals for the optical scanner 14 and inparticular for the drives via at least one digital signal processor,where said digital signal processor which has access to a memory (notshown) or an area of a memory 28 of the control and analyzer device 8.

The deflected illumination beam 4 emerging from the deflection section 5is focused by the microscope lens 16 in a focus 17 on or in the sample2. For selecting the position of the focus 17 in z-direction, i.e., inthe depth of the sample, the position of a sample table 34 can beadjusted in the z-direction in a known manner by means of an integrateddrive connected to the control and analyzer device 8 throughcorresponding signals of the control and analyzer device 8.

The focus 17, i.e., more precisely the detection radiation 18 emitted bythe focus 17, e.g., fluorescent radiation excited by the illuminationradiation in the sample 2 is mapped by the mapping optics 6, whichincludes the microscope lens 16 that becomes larger along a detectionbeam path, the beam splitter 13 and a detection optics 19 mapping thefocus 17 confocally on the acquisition device 7.

The microscope optics 16 comprises a tube lens 20 and a microscopeobjective 21.

The beam splitter 13 is designed to allow at least a portion of theillumination beam 4 to pass through and deflect the detection radiation18 to the detection lens 19.

The deflection section 5 which descans the detection radiation 18 isarranged between the microscope lens 16 and the beam splitter 13.Descanning is understood to mean that the deflection section isdeflected by the focus 17 traveling on or in the sample according to thedeflection of the illumination beam 4 of the radiation emitted into afixed section of the detection beam path, in particular to the beamsplitter 13, so that the section of the detection beam path between thesample 2 and the deflection section 5 is pivoted according to themovement of the section of the illumination beam path between thedeflection section 5 and the sample 2, while the remainder of thedetection beam path remains unchanged, however.

The detection radiation 18 from the focus 17, fluorescent radiationexcited by the illumination radiation or illumination radiationreflected back from the focus 17 by the sample 2 passes through themicroscope lens 16 and the deflection section 5, is deflected by thebeam splitter 13 and then goes to the confocal detection lens 19 whichcomprises a detection lens or lens group 22 and a fine hole aperture,i.e., pinhole aperture 23. The detection lens and/or lens group 22 isdesigned and arranged in such a way that detection radiation coming outof the focus 17 is focused in the orifice of the pinhole aperture 23 andpasses through it, but radiation from other areas of the sample 2essentially cannot pass through the pinhole aperture 23. In this waythere is confocal mapping of only one layer of the sample 2 runningessentially parallel to the x-y plane with the focus 17. The size of theopening in the pinhole aperture 23 determines, among other things, thedepth resolution of the laser scanning microscope 1 in a known way.

Behind the pinhole aperture 23 there is optionally a filter (not shownin FIG. 1) for the detection optics 19, such that the filter does notallow unwanted radiation components to pass through or they pass throughonly to a greatly diminished extent or in the case of fluorescence teststhe illumination radiation reflected back out of the focus 17 is notallowed to pass through or passes through only to a greatly diminishedextent.

The detection radiation 18 then goes to the acquisition device 7. Theacquisition device 7 has a detection element 24, namely in the exampleshown here a photomultiplier on which the focus 17 is mapped and whichdetects the detection radiation which has passed through the pinholeaperture 23, generating and emitting detection signals.

The acquisition device 7 and in it in particular the detection element24 are connected via a signal connection to the control and analyzerdevice 8 which serves to trigger the deflection section 5 and drive thesample table 34 and also to analyze the signals of the acquisitiondevice 7, more precisely the image data acquisition signals and/ordetection signals of the photomultiplier 24 and to form correspondingimages. These images may be displayed with the display device 9, namelya color monitor in the present example, which is connected to thecontrol and analyzer device 8 via a signal connection.

The control and analyzer device 8 is designed as a data processingdevice and has an acquisition interface 26 for the acquisition device 7and/or its photomultiplier 24, a graphics interface 27 connected to thedisplay device 9, a memory 28 for storing data and instructions of acomputer program and a microprocessor 29 that is connected to theinterfaces 26 and 27 and to the memory 28 and at least partiallyexecutes the method described below in execution of the instructions.Additional instructions of a computer program that allows operation andcontrol of the laser scanning microscope, e.g., by input of a scanningspeed and a scanning amplitude via a graphical user interface, is savedin the memory 28 or, more precisely, in a nonvolatile portion of thememory 28.

As an alternative the program can also be stored on a data medium suchas a CD which can be read via a CD drive (not shown) that is connectedto the microprocessor 19.

The microprocessor 29 controls, among other things, the scan controlunit 35, namely the signal processor of the scan control unit 35 in theexample. In other exemplary embodiments, it is possible for themicroprocessor 29 to form a part of the scan control unit 35, to whichend it is then programmed accordingly. The scan control unit and thecontrol and analyzer device are then partially integrated.

In addition, devices for synchronization of the acquisition ofintensities by the photomultiplier 24 with the position of the scanner14 are also provided in the control and analyzer device 8, these devicesbeing implemented in part by the microprocessor 29 and in additionoptionally comprising other components (not shown in FIG. 1) with whichthose skilled in the art are familiar.

Images of a layer in the sample 2, whose layer thickness is determinedby the size of the opening of the aperture 23 and by the properties ofthe mapping optics 6, are reproduced by a rectangular arrangement ofimage elements, i.e., pixels 25, which have been shown with a reducednumber of pixels in comparison with the actual arrangement, as indicatedschematically in FIG. 7 for the sake of clarity.

The acquisition of a recorded image of a layer takes place as follows:the radiation source 10 emits the illumination beam 4 which is guided bymeans of the scanner 14 line-by-line in x-direction over the sample 2.Detection radiation emitted from there is sent via the microscope optics16 to the deflection section 5 where it is descanned. After deflectionby the beam splitter, the confocal components are acquired by theacquisition device 7 and corresponding signals are transmitted to thecontrol and analyzer device 8. Then after the end of acquisition of aline of pixels, the focus moves in the y-direction, superimposed on themovement in x-direction, whereupon the next line in x-direction can bedetected, e.g., by reversing the direction of travel.

The movement in x-direction takes place periodically. The setpointmovement is depicted as an example in FIG. 3, where s denotes theposition of the focus in x-direction and t denotes the time. Ideally thefocus moves in x-direction on its path from a first end position to asecond end position and back into the first end position, i.e.,periodically, whereby the movement from one end position into the othertakes place in a straight line and uniformly at least in the effectivearea that is used for acquisition of an image and has the width 2A inFIG. 3. This movement is characterized by the two straight lines 39 inFIG. 3.

In fact however, the focus cannot be accelerated with infiniteacceleration from one of the end positions or decelerated with infinitedeceleration in approach to the end position. Therefore, there followsat first at the beginning of a period, i.e., starting from a first endpoint, namely the point in time 0 in FIG. 3, there is an acceleration toa subsequently desired speed which is reached at time t₁. In thefollowing period of time until time t₂ there is a linear and uniformback-and-forth movement during which image data is acquired. Then in thetime between t₂ and t₃ the drive 30 and the mirror 31 and thus the focus17 are decelerated to the speed 0 which is reached in the second endpoint. From there the drive 30 and the mirror 31 are accelerated in thereverse direction until time t₄ at which the same speed is reached againas in the forward movement. At the same time in the period of timebetween t₂ and t₄ by triggering the drive for the movement iny-direction, the focus is moved in the y-direction for a predeterminedperiod of time and/or by a predetermined distance so that in the reversemovement the next line of an image can be acquired. In the followingperiod of time t₅ the linear uniform reverse movement takes place,during which image data can be acquired again. Between times t₅ and t₆the drive 30 and the mirror 31 are decelerated again, whereby againthere is a movement of the focus 17 in y-direction to the next line. Theareas covered between times t₁ and t₂ or t₄ and t₅ thus constitute aneffective area for image acquisition, and the width 2A is twice theamplitude A of the scanning movement.

During the linear uniform back-and-forth movement, intensities and/orenergies of the detection radiation are determined at constant intervalsof time by the photomultiplier 24 and corresponding signals emitted bythe photomultiplier 24 are input via the interface 26, assigned to thepixels and saved in the memory 28 of the control and analyzer device 8.This means that the number of pixels of a line is defined by the numberof points in time at which the intensity of the detection radiation isacquired. The intensity values acquired and assigned to the pixels aresaved as a matrix with elements I(i, p) whereby the index p denotes theposition of the line i. The distance of the areas depicted by the pixelsin the image from one another is obtained as the product of the speed ofthe focus 17 and the interval of time Δt between the acquisition ofsuccessive intensities. Since the time continues to run in the periodicmovement but with the movement in the x-direction each location ispassed over twice in a period, so the intensities are saved directly inthe control and analyzer device 8 in accordance with their pixels andthus assigned to locations. This is illustrated in FIG. 7, whereintensities for the pixel with the index j=1 nearest the first endposition, for example, are acquired at points in time t₁ and t₂ andintensities for the pixel with the index j=p_(max) nearest to the secondend position (in example 512) are acquired at points in time t₃ and t₄and saved directly accordingly.

The trigger signals corresponding to the setpoint movement are generatedby a synthesis of frequency components. To do so, first the setpointmovement is represented as a Fourier series with a period of fundamentalfrequency f corresponding to the x movement. For the triangular movementillustrated in FIG. 3, the following approximate representation isobtained for the deflection s_(setpoint)(t) of the focus 17 throughappropriate deflection of the mirror 31 as a function of time t:${{{s_{{set}\quad{point}}(t)} \approx {\sum\limits_{k = 1}^{N}{s_{k\quad{set}\quad{point}}{\cos( {{2{\pi \cdot k \cdot f \cdot t}} + \varphi_{k\quad{set}\quad{point}}} )}}}} = {\frac{8}{\pi^{2}}{\sum\limits_{k = 1}^{N}{\frac{a_{k\quad{set}\quad{point}}}{k^{2}}{\cos( {{2{\pi \cdot k \cdot f \cdot t}} + \varphi_{k\quad{set}\quad{point}}} )}}}}},$where s_(ksetpoint) and a_(ksetpoint) denote amplitude coefficients andφ_(ksetpoint) denotes phase coefficients for the frequency componentwith the frequency k·f. The greater the positive natural number in N>1is selected to be, the more accurately the Fourier sum represents thesetpoint movement. In practice, N is selected so that a predeterminedprecision in the representation is achieved. The curve in FIG. 3represents the Fourier sum for N=1.

The scan control unit 35 generates the trigger signals that are suppliedto the position regulator 33. The trigger signals are preferablygenerated so that the mobile beam path section and/or the focus 17executes the setpoint movement in the best possible approximation. To doso a predistortion (see FIG. 4) is performed, taking into account theinfluence of all the intermediate components, e.g., the triggerelectronic system including the drive and the mechanical and opticalcomponents such as mirrors, etc. in the form of an optical transferfunction U. When using the Fourier representation of the setpointmovement as described above, the optical transfer function is given bytwo components, namely an amplitude component U_(A) and a phasecomponent U_(p) which depend on the frequency. Thus, in a representationof the trigger signals corresponding to a setpoint movement as a Fourierseries and/or sum:${s(t)} \approx {\sum\limits_{k = 1}^{N}{s_{k}{\cos( {{2{\pi \cdot k \cdot f \cdot t}} + \phi_{k}} )}}}$with Fourier amplitudes S_(k) and Fourier phases φ_(k) which areobtained from the corresponding Fourier components of the setpointmovement, and a corresponding representation of the movement of thefocus:${s_{focus}(t)} \approx {\sum\limits_{k = 1}^{N}{s_{{focus},k}{\cos( {{2{\pi \cdot k \cdot f \cdot t}} + \phi_{{focus},k}} )}}}$the following equations hold:s _(focus,k) =s _(k) ·U _(A)(k·f) andφ_(focus,k)=φ_(k) +U _(p)(k·f),where U_(A)(k·f) is the amplitude component of the transfer function andU_(p)(k·f) is the phase component for the frequency kf.

In order for s_(focus) to actually follow the setpoint path, thepredistortions _(v,k) =s _(k) /U _(A)(k·f) andφ_(v,k)=φ_(k) −U _(p)(k·f),is calculated and the position regulating device is triggered withtrigger signals according to the Fourier series and/or sum withpredistorted amplitudes sV,k and predistorted phases φ_(v,k) of thefrequency components. The position regulating system 33 and the positionsensor 32, the function of which is taken into account in the transferfunction, then ensure an accurate triggering of the drive so that therequired position is in fact reached (see FIG. 4). With a complete andaccurate knowledge of the transfer function, then the focus 17 willexecute the setpoint movement with exactly linear systems.

The optical transfer function, however, is known at least theoreticallyfor a laser scanning microscope based on its design. In fact, the actualtransfer function usually deviates from the theoretical, e.g., due toambient influences and drift in the properties of the components used sothat by triggering according to a setpoint movement, an actual movementthat may deviate from the setpoint movement is achieved. To eliminatethe deviations, the optical transfer function must be determined. Thismay be accomplished with the following determination and/or calibrationmethod, for example.

With this calibration method which is depicted schematically in FIGS. 5and 6, a reference sample 2 is used as the sample having structures thatcan be detected with the laser scanning microscope 1. This presupposesthat at least the forms of the structures and their position in relationto one another are known. An image of the reference sample is createdusing a setpoint test movement from which the actual positions of thestructures are ascertained and after ascertaining the setpointpositions, they are compared with the setpoint positions of thestructures in the figure. On the basis of the deviations between thesetpoint positions and the actual positions, corrections for the opticaltransfer function and/or its frequency components can then beascertained so that an improved, preferably accurate transfer functionis obtained when they are used.

In the present exemplary embodiment, a sample is used with a periodicline grid (see FIG. 8) in which the lines 36 are at a distance from oneanother, said distance being greater than twice the distance of thefocus positions on the reference sample 2 in two successive acquisitionsof the intensity and/or energy during a movement in x-direction.

After positioning the reference sample 2 on the sample table 34, thecontrol and analyzing device 8 together with the scan control unit 35performs the following method, to which end the microprocessor 29 andthe scan control unit 35 and/or a digital signal processor thereinprocess instructions of one or more computer programs saved in thememory 28 in the control and analyzer device 8.

It is assumed here that values of a transfer function are already onhand and have them saved. If there are no known specific values for thescanner, it may initially be assumed, for example, that the amplitudecomponents U_(A) for all the frequencies in question assume a value of 1and the phase components Up assume a value of 0.

First in block S10 a reference deviation function is ascertained,reflecting the deviations between the setpoint positions and the actualpositions and thus the actual movements, and the reference deviationfunction is determined using the saved instantaneous transfer function.

The steps of block S10 are depicted more precisely in FIG. 6.

First, the control and analyzer device 8 reads in step S12 the distancebetween the lines 36 on the reference sample via a graphical userinterface. In other exemplary embodiments, the distance between thelines may also be saved in a nonvolatile portion of the memory 28 andthen the value is read out of the memory 28.

In the next step S14 the control and analyzer device 8 also obtains thefrequency f and the amplitude A of the setpoint scanning movement,likewise via the graphical user interface. The frequency f is thefundamental frequency of the Fourier sum to be formed subsequently. Forthe case when scanning programs, i.e., combinations of scanning speedsand amplitudes, have already been predetermined, it is sufficient forthe user to select the corresponding scanning program and then thecontrol and analyzer device 8 reads out the corresponding values fromthe nonvolatile portion of the memory 28.

In the following step S16 the control and analyzer device 8 alsoacquires the number N of the frequency components to be used, which canbe acquired like the parameters of the scan program. The number N shouldadvantageously be selected so that the frequency N·f is less than apredetermined cut-off frequency which may be selected as a function ofthe properties of the position regulating system 33 and the drive 30,for example. In addition the number N should not be greater than apredetermined maximum number which is selected as a function of thedesired precision in the scanning movement. The number N is preferablybetween 10 and 50 so as to achieve the accuracy required for laserscanning microscopes.

In step S18 the control and analyzer device 8 sets initial values forthe parameters that are used for the control or regulation and are to becorrected, i.e., the amplitude s_(k) and the phases φ_(k) for thefrequency components for frequencies k f, where k=1, . . . N. As alreadymentioned, the scan frequency f is the fundamental frequency, while thefrequencies k f are the harmonic freqeuncies for k>2. From theparameters that are used and are to be corrected, it then determines thetrigger signals using the optical transfer function as described above,and performing the predistortion; these trigger signals are thendelivered to the position regulating system 33 in step S20 which followsto acquire an image of the reference sample 2. The optical transferfunction is saved as a table in the nonvolatile portion of the memory atpredetermined interpolation points, whereby either the values used lastor estimated values may be used. If values of the transfer function atthe required frequencies have not been saved, they may be obtained byinterpolation.

In step S20 by periodic scanning of the reference sample 2 according tothe calculated trigger signals at least in x-direction, an image of thereference sample 2, i.e., an image of at least a section of thereference sample 2 is acquired. The reference sample 2 is oriented sothat the grid lines are orthogonal to the x-direction in goodapproximation. The scan amplitude is selected as a function of thedistance between the lines and the number N of the frequency componentsso that at least as many grid lines are acquired as there are frequencycomponents being used. The image is then saved in the control andanalyzer device 8 in the memory 28 as described above. In anotherexemplary embodiment, it is also possible to acquire the image withy-deflection between the movement in x-direction.

FIG. 8 shows a detail from the reference sample 2 and the imageacquired. The lines 36 on the reference sample 2 are not acquired at thelocations where they should be located. Instead, the lines are acquiredduring the forward movement along the lines 37 with large dots andduring the reverse movement on the lines 38 with small dots, resultingin deviations As in the positions.

To be able to quantitatively detect the deviations, first the setpointpositions of the structures, i.e., grid lines in the image acquired aredetermined. This does not require positioning the sample with extremelyhigh precision on the slide stand.

Therefore in step S22 the actual position of at least one firststructure of the reference sample is determined from the image, namelyin the example of the line which was determined during the forwardmovement in the image as being the center of the scan line, i.e., thecenter of the effective area, i.e., the next to be determined. Then theactual position of a line in the image closest to the line ascertainedin the forward movement is ascertained during the reverse movement. Thenthe average of the actual positions thus ascertained in the forward andreverse movements is calculated and saved as the calculated setpointposition of the line.

In step S24 the setpoint positions of the remaining lines of thereference sample 2 are ascertained in the image and saved, starting fromthe actual position of the middle line using the known spacing of thestructures and/or lines from one another.

In step S26 the actual positions of the remaining lines and theirdeviations from the setpoint positions are then determined separatelyfor the back-and-forth movements. To do so first using known methods theactual positions of the structures and/or lines in the image areascertained. In this process, the actual positions are allocated topixels, i.e., to the numbers thereof and the back-and-forth movements.Then the deviation As between the setpoint position and the actualposition is calculated for the lines and saved. Deviations are depictedin FIG. 9 a for the forward movement and FIG. 9 b for the reversemovement as a function of pixel number p as an example. For a betterunderstanding of the following steps as well, the times when the valueswere acquired are also indicated below the pixels. The actual positionsof the lines can be determined with sub-pixel accuracy by interpolationof the intensity variations.

In step S28, a deviation function representing the deviations As as afunction of time t during the movement and not as a function of pixelnumber p is then determined. Such a function which is given only by thevalues at the acquisition points in time is represented partially and asan example in FIG. 10. The deviation values thereby acquired andassigned to the pixels are rearranged so that they are arranged in orderof their acquisition time. The values in the five different periods ofthe test movement are handled different here.

In the time between 0 and t₁ when the drive 32 and the mirror 33 areaccelerated from a standstill at the reversal point and/or at the firstend position of the movement to the desired scanning speed, nodeviations are ascertained. Therefore no pixels need be depicted forthis period of time.

In the forward movement between the times t₁ and t₂ the scanner 14 movesthe focus 17 at a speed that is at least approximately constant,depending on the quality of the current calibration, whereby theintensity values and/or energy values are acquires at constantintervals. These values are assigned in succession to pixels with anascending ordinal number for the y-direction. Therefore, the acquisitiontimes t_(p) are assigned as times to the pixels and/or the deviations.These are obtained from the ordinal number p of the pixel multiplied bythe constant interval of time Δt between the acquisition of immediatelysuccessive intensity values with respect to the time t₁:t _(p) =pΔt+t ₁

Therefore, no rearrangement is required.

No deviations are determined between the times t₂ and t₃, i.e., thereversal time for braking to the second end position and accelerationfrom the second end position in the direction of the first end position,so that here again no intensities need be detected.

In the reverse movement between the times t₃ and t₄, the scanner movesthe focus 17 at a speed that is at least approximately constant inaccordance with the quality of the prevailing calibration, whereby theintensity and/or energy values are obtained at constant intervals.However, because of the reverse movement, these are assigned to pixelsin chronological succession with descending ordinal numbers for they-direction. Therefore, the acquisition time t_(p) is assigned to eachpixel for the reverse movement, this acquisition time being obtained,for example, from the maximum number p_(max) of pixels acquired, theordinal number p of the respective pixel and the interval of timebetween the acquisition of the intensity for two successive pixels andtime t₃ as follows:t _(p)=(p _(max) −p)Δt+t ₃

These times are assigned to the deviations for the pixels in the reversemovement, whereby they are rearranged according to the times t_(p) byreversing their order.

Again no deviation values are ascertained in the period of time betweentimes t₄ and t₅ during which the drive 32 and the mirror 33 aredecelerated in the first half of the reversing movement, then reaching aspeed of zero in the first end position.

The result of this step is a reference function, i.e., curve for thedeviations in the actual test movement from the setpoint test movement,determined on the basis of a bidirectionally recorded image of thereference sample with known structures in their relative positions inrelation to one another. The interpolation points at which thedeviations, i.e., the values of the deviation function, are given, arenot equidistant here and instead gaps caused by the reversal periodsoccur here.

In step S30 a test frequency is set at the fundamental frequency and aparameter k_(test) is set at the value 1.

In step S32 a frequency component Z(t) with the test frequency k_(test)fis added to the trigger signal, the amplitude δs preferably beingselected so that, first of all, it is definitely smaller than the valueof the amplitude s_(ktest), but secondly, a change in the deviation Δsinduced by this change can still be detected with only a slight error.The phase φ_(z) of the added harmonic function is advantageouslyselected so that the resulting deviation curve is approximatelysymmetrical with the midpoint of the back-and-forth movement. Thisfacilitates the analysis of the response to the change.

Then the steps S18 through S28 are performed again with the alteredfrequency components as output values for determining the initialvalues, in which case a modified deviation function is obtained. Thesesteps are represented by a block S34 in FIG. 5.

In step S36 the differential function of the deviation functionascertained in block S34 and the reference deviation function obtainedin block S10 is then ascertained. The difference is the response of thescanner to the additional excitation applied, which is described by thefrequency component Z(t). Assuming a linear system, the response is inturn a harmonic function with the test frequency but with an alteredamplitude and phase in comparison with the frequency component of thesetpoint test movement.

Therefore, in step S38 the amplitude A_(m) and the phase P_(m) of thedifferential function and/or the response function are ascertained byadjustment with a harmonic function.

In step S40, the transfer function of the scanner is then corrected byaltering the frequency component of the transfer function for the testfrequency. To do so, the amplitude component U_(A) and the phasecomponent U_(p) are ascertained on the basis of the amplitude A_(m) andthe phase P_(m) of the response curve and the amplitude δs and phaseφ_(z) and are saved instead of the values saved previously. In anotherexemplary embodiment, the amplitude component U_(A) and the phasecomponent U_(p) may be ascertained on the basis of the amplitude andphase of response curve and the amplitude and phase may each beascertained as a sliding average over several measurements, e.g., fouror five measurements,

In step S42 the value ktest is then incremented by 1. If the resultingvalue is smaller than N, then the test frequency is set at the valuek_(test)f and the method is continued with step S32. Otherwise thecorrection method is terminated.

In this way the transfer function of the scanner is corrected over theentire frequency range used so that the deviations between the setpointmovement and the actual movement of the focus 17 are reduced, namelybeing minimized in the ideal case. This correction is at the same timeto be regarded as a determination of the optical transfer function.

Immediately after the determination of the optical transfer function, adrive unit transfer function is determined; when using the Fourierrepresentation mentioned above, this transfer function also has anamplitude component U_(A,e1) and a phase component U_(P,e1).

This drive unit transfer function is determined by the scan control unit35 and/or the signal processor contained therein as follows.

For the fundamental frequency and each of the harmonic frequencies used,a harmonic trigger voltage of a known amplitude is generated via thescan control unit 35 and sent to the drive 30. The check-back signal ofthe position sensor 31 is ascertained, this signal also being a harmonicfunction of the same frequency, but usually with a different amplitudeand a different phase. The ratio of the amplitude of the check-backsignal to the amplitude of the trigger voltage is used as the frequencycomponent of the amplitude component U_(A,e,1), and the differencebetween the phase of the trigger signal and the phase of the check-backsignal is used as the frequency component of the phase componentU_(P,e1). The values ascertained for the respective frequencies, likethe values of the optical transfer function, are saved in a table as afunction of the frequency.

In the correction method according to a first preferred embodiment ofthe present invention, a correction is now performed starting from thestored optical transfer function and the stored drive unit transferfunction.

Therefore, the scan control unit 35 is programmed so that, afteractivation of the scan control unit and after a predetermined warm-uptime of approximately five minutes since turning on the scanner 14 inthis example, it ascertains instantaneous valuesU_(A,e1 akt)(k_(test)·f) and U_(P,e1 akt)(k_(test)·f) for the drive unittransfer function for a saved frequency k_(test)·f as described above.In doing so, it has a timing device implemented via software to monitorthe course of the predetermined warm-up time.

In the next step, the optical transfer function is then correctedaccording to the following equations in which, for the sake of clarity,the optical transfer function now appears as U_(optical) instead ofsimply U to facilitate a differentiation:${{U_{A,{optical},{corr}}( {k_{test}f} )} = {\frac{U_{A,{optical}}( {k_{test}f} )}{U_{A,{el}}( {k_{test}f} )}{U_{A,{el},{akt}}( {k_{test}f} )}\quad{and}}}\quad$U_(A, optical, corr)(k_(test)f) = U_(P, optical)(k_(test)f) − U_(P, el)(k_(test)f) + U_(P, el, akt)(k_(test)f).

The values U_(A,optical)(k_(test)·f) and U_(P,optical)(k_(test)·f) savedlast are then replaced by the values thus obtained in memory 28.

Next the index k_(test) is incremented by 1. When it exceeds the maximumvalue N, k_(test) is reset at 1 so that in the next correction, acorrection of the fundamental frequency is performed.

Over a period of time, this therefore yields a correction for allfrequencies.

In another preferred embodiment, the programming of the scan controlunit 35 and optionally the control and analyzer device 8 is modified insuch a way that the determination step and the correction step areperformed for several preselected frequencies. The frequencies, e.g.,five frequencies with approximately 30 harmonic frequencies, aredistributed uniformly over the entire frequency range used. By means ofan interpolation model, e.g., equalizing splines in a double logarithmicdisplay, values for the frequencies in between can then be interpolated.When repeating the correction, five other frequencies, each incrementedby the fundamental frequency, for example, are used. In this way abetter correction can be achieved each time the method is performed.

In other preferred embodiments, it is also possible to use differentmodels in which a functional form of the frequency dependence ispredetermined by a parameterized function. The parameters may then bedetermined by fitting.

In another preferred embodiment, through appropriate programming of thescan control unit 35 and the control and analyzer unit 8, triggering ofthe correction method may be started manually as needed either as analternative or in addition.

In yet another preferred embodiment of the method, in contrast with thefirst exemplary embodiment, the transfer function is not corrected andinstead the parameters S_(k) and φ_(k) representing the setpointmovement and/or the corresponding trigger signals are corrected. To doso, the programming of the scan control unit 35 is revised accordinglywhile the other components of the scanner and the laser scanningmicroscope remain unchanged so that the statements made regarding theprevious exemplary embodiments are also applicable here. The equationsfor the corrected Fourier amplitudes S_(ktest,corr) and Fourier phasesφ_(ktest,corr) can then be written as follows:$S_{{k\quad{test}},{corr}} = {\frac{U_{A,{el}}( {k_{test}f} )}{U_{A,{el},{akt}}( {k_{test}f} )}s_{k\quad{test}}\quad{and}}$ϕ_(k  test, corr) = ϕ_(k  test) + U_(P, el)(k_(test)f) − U_(P, el, akt)(k_(test)f),where s_(ktest) and φ_(ktest) denote the parameter values saved beforethe correction and ascertained at the same time and saved at the sametime as U_(A,e1) and U_(P,e1).

The triggering of the scanners is then performed with the parameterss_(ktest,corr) and φ_(ktest,corr) starting at this point in time. Theoriginal parameters s_(ktest) and φ_(ktest) of the trigger signals andthe respective parameters U_(A,e1) and U_(P,e1) of the transfer functionare kept in the memory 28 for correction of other frequency componentsand/or for renewed subsequent correction of the same frequencycomponents.

1. Method for correcting a control of an optical scanner (14), having abeam deflecting element (31) for deflection of a beam of opticalradiation and a drive unit (30, 30′, 35) for moving the beam deflectingelement (31), said drive unit moving the beam deflecting element (31)according to trigger signals generated by using at least one parameterfor control or regulation and/or using a transfer function, preferablyoptical, from a predetermined-setpoint movement, wherein in adetermination step at least one instantaneous value of a drive unittransfer function is determined for at least one predeterminedfrequency, said drive unit transfer function reflecting the response ofthe drive unit (30, 30′, 35) to trigger signals determined from apredetermined setpoint movement or a change in a setpoint movement andin a correction step at least one parameter and/or the transfer functionis corrected as a function of the instantaneous value of the drive unittransfer function.
 2. Method according to claim 1, wherein theinstantaneous value of the drive unit transfer function is ascertainedin the determination step in that an actual position of a drive (30) ora mechanical coupling element (30′) of the drive unit (30, 30′, 35) isascertained as a function of the setpoint movement and/or the change inthe setpoint movement or the trigger signals ascertained therefrom. 3.Method according to claim 1, wherein the determination step and thecorrection step are repeated at predetermined intervals of time. 4.Method according to claim 1, wherein the determination step and thecorrection step are performed with each activation of the scanner (14)or at a predetermined interval of time after each activation.
 5. Methodaccording to claim 1, wherein the determination step and the correctionstep are repeated and the frequency used in these steps is change witheach repetition.
 6. Method according to claim 1, wherein thedetermination step and the correction step are performed for severalpredetermined frequencies.
 7. Method according to claim 1, wherein adeviation in the instantaneous value of the drive unit transfer functionfrom a corresponding value of the drive unit transfer function savedpreviously, said value having been ascertained within a predeterminedinterval of time before or after ascertaining the value of the opticaltransfer function currently being used at the frequency is determined inthe correction step and the deviation is used for the correction. 8.Method according to claim 1, wherein the correction for at least onefrequency is performed in the correction step and a model for thefrequency dependence of the parameter to be corrected and/or thetransfer function to be corrected is determined for correction for otherfrequencies.
 9. Optical scanner having a beam deflecting element (31)for deflection of a beam of optical radiation and a drive unit (30, 30′,35) for moving the beam deflecting element (31) which moves the beamdeflecting element (31) according to trigger signals, and a scan controlunit (35) that is connected to the drive unit (30, 30′) for transmittingtrigger signals for control of the drive unit (30, 30′) by generatingtrigger signals which are designed so that, by using at least oneparameter that is used for control or regulation and/or one transferfunction, preferably optical, of a predetermined setpoint movement,corresponding trigger signals can be generated, and for correction ofthe control of the scanner (14) in a determination step at least oneinstantaneous value of a drive unit transfer function that reproducesthe response of the drive unit (30, 30′) to trigger signals ascertainedfrom a predetermined setpoint movement or a change in a setpointmovement, can be ascertained for at least one predetermined frequency,and in a correction step the at least one parameter and/or the transferfunction can be corrected as a function of the instantaneous value ofthe drive unit transfer function.
 10. Optical scanner according to claim9, wherein the drive unit (30, 30′) has a stepping motor for moving thebeam deflecting element (31) and/or a position sensor (32) for acquiringthe position of a motor or a mechanical coupling element of the driveunit (30, 30′) and the scan control unit (35) is further designed sothat in the determination step the position of the stepping motor or aposition signal of the position sensor (32) is used to ascertain theinstantaneous value of the drive unit transfer function.
 11. Opticalscanner according to claim 9, having a timer device which triggers thescan control unit (35) repeatedly at predetermined intervals of time sothat it executes the determination step and the correction step. 12.Optical scanner according to claim 9, wherein the scan control unit (35)is further designed so that the determination step and the correctionstep are performed with each activation of the scanner (14) or at apredetermined interval of time after each activation.
 13. Opticalscanner according to claim 9, wherein the scan control unit (35) isfurther designed so that it repeatedly executes the determination stepand the correction step and changes the frequency used in these stepswith each repetition.
 14. Optical scanner according to claim 9, whereinthe scan control unit (35) is further designed so that the determinationstep and the correction step are performed for several predeterminedfrequencies.
 15. Optical scanner according to claim 9, wherein the scancontrol unit (35) is further designed so that a deviation in theinstantaneous value of the drive unit transfer function from acorresponding value of the drive unit transfer function saved previouslyis determined in the correction step, said previously saved drive unittransfer function having been ascertained within a predeterminedinterval of time before or after the determination of the value of theoptical transfer function currently being used at the frequency, andthis deviation is used for the correction.
 16. Optical scanner accordingto claim 9, wherein the scan control unit (35) is further designed sothat in the correction step the correction for at least one frequency isperformed and a model for the frequency dependence of the parameters tobe corrected and/or the transfer function to be corrected is determinedfor other frequencies for the correction.
 17. Laser scanning microscopehaving an optical scanner according to claim 9.